In recent years, it is becoming increasingly common for fuel injection type internal combustion engines to be used in vehicles and the like. Among these internal combustion engines are those in which a throttle valve (i.e., a diaphragm valve) is provided on an upstream side of an air intake manifold, and a fuel injection valve and an air flow rate sensor are provided on a downstream side of the throttle valve (see, for example, Japanese Examined Patent Application, Second Publication No. H04-15388). Air flow rate signals output from the air flow rate sensor are input into a control circuit and the fuel injection rate is computed in accordance with the running state of the internal combustion engine. Operations of the fuel injection valve are then controlled by outputting fuel injection rate signals that are based on the computed fuel injection rate from the control circuit.
In order to improve the fuel efficiency and response of the internal combustion engine, it is desirable to detect the air intake quantity (i.e., the quantity of air taken into the internal combustion engine) each time air is taken in, and to then perform the optimum fuel injection based on this air intake quantity. However, in a conventional structure, when the interior of the air intake manifold is in a negative pressure state, the air flow rate sensor ends up measuring an air flow rate which is obtained by adding the air that is taken into the internal combustion engine to the air that fills the interior of the air intake manifold. Therefore, the drawback arises that the optimum fuel injection cannot be performed particularly when the throttle valve is first opened.
The present invention was conceived in view of the above described circumstances and it is an object thereof to provide an air intake apparatus for an internal combustion engine that makes the optimum fuel injection possible by accurately measuring an air intake quantity even when a throttle valve is first opened.